新建一个基类shape类是一个表示形状的抽象类，area( )为求图形面积的函数。请从shape类派出矩形类(rectangle)、三角形类(triangle)、圆类（circles）、并给出具体的求面积函数。-Create a new class is a base class shape represents the shape of an abstract class, area () function for the sake of the graphic area. Please send from the rectangular shape class class (rectangle), triangles class (triangle), round class (circles), and give a specific function for the area.